In recent years, thermally assisted magnetic recording (TAMR) and microwave assisted magnetic recording (MAMR) are studied for improving the recording density of hard disk drive (HDD). Any of them uses a granular magnetic recording medium in which fine magnetic grains are surrounded by a non-magnetic matrix. Typically 10-20 magnetic grains constitute a recording unit (reversed magnetic domain) as 1 bit. However, when the magnetic grains are made finer so as to make the reversed magnetic domain smaller, they cause a problem of large deviation in magnetic properties. On the other hand, decreasing the number of magnetic grains per 1 bit to make the reversed magnetic domain smaller has a problem of small signal to noise ratio (SNR). These problems make it difficult to increase the recording density by using TAMR and MAMR technologies.
A technique called three-dimensional recording or volume recording is studied to solve these problems. In the volume recording, the recording density per unit area is increased by recording information along the film thickness direction as well as in-plane direction of the magnetic recording medium. That is, information is also recorded in the additional layers under the topmost recording layer.
However, the volume recording method has following three problems when it is applied to a current HDD system.
The first problem is low recording resolution.
High density magnetic recording on the granular medium needs narrow magnetization transition region, which is an interface between two magnetic domains. Therefore, a recording magnetic field generated by a recording magnetic pole needs to be steeply decreased along down track direction (needs to have large magnetic field gradient).
Generally, the magnetic field gradient decreases with increasing a distance from the recording magnetic pole along the film thickness direction. Therefore, current HDD system is designed to have the recording layer as thin as possible and the flying height (distance between the recording head and the medium) as small as possible in order to make magnetic field gradient as large as possible. For the case of the conventional volume recording, high density recording can be achieved only on the topmost layer close to the recording magnetic pole, but is difficult on underlying layers far from the recording magnetic pole. The storage capacity of the whole magnetic recording medium can be increased to some extent but not by twice compare to the case of current single layer recording.
This problem can be solved to some extent by using a bit-patterned recording medium with multilayer structure. In the bit-patterned medium, the recording layer is etched into a magnetic dot having a size of 1 bit. Since it is not necessary to form the magnetization transition by the recording magnetic field, high density recording is achieved even in the underlying layers where the magnetic field gradient is not so large.
However, low magnetic field gradient increases the probability of accidental reversal of adjacent dots, and therefore the problem cannot be completely solved.
The second problem is low reproduction resolution.
In principle, a leakage magnetic field from each bit on the recording medium decreases in intensity with distance from the recording magnetic medium, and also the change in the magnetic field intensity on the magnetization transition region decreases with distance. In other words, a spatially blur reproduced signal is generated.
This phenomenon is a fundamental property as in the case of the recording magnetic field. Accordingly, current HDD system requires recording layer as thin as possible and the flying height as small as possible. Therefore, as in the case of the first problem, high density reproduction can be achieved only on the topmost layer close to the reproducing head for the current volume recording system.
This problem cannot be solved even using the bit-patterned medium. No matter how clearly the magnetization is spatially changed in underlying layers of the bit-patterned medium, it is recognized as a blur spatial magnetization change by the reproducing head far from it.
The third problem is a decrease in signal intensity due to the superimposing of multiple magnetization states.
Assume that a volume recording medium is composed of two layers (first layer and second layer) and a magnetization of the first layer is M1 and its thickness is t1, a magnetization of the second layer is M2 and its thickness is t2, and M1×t1≠M2×t2. When the combination of directions of the magnetizations of the first layer and the second layer are (up, up), (up, down), (down, up), (down, down), reproduced signal intensity is proportional to M1×t1+M2×t2, M1×t1−M2×t2, −M1×1+M2×t2, −M1×t1−M2×t2 respectively, and all of the cases can be distinguished by the amount of the reproduced signal in principle.
However, for the case of magnetized in different directions (the cases of M1×t1−M2×t2 and −M1×t1+M2×t2), the reproduced signal becomes lower than that in the case of conventional single layer recording. This means a decrease in SNR. High density recording is necessary just because the SNR in the conventional single layer recording reaches its lowest limit. High density recording technology that decreases SNR does not make sense. Furthermore, when the thickness of each layer is reduced to solve the aforementioned two problems (low recording resolution and low reproduction resolution), the problem of the low SNR becomes worse.